The present invention relates to an electrical resistor and more particularly, to a variable resistor of semi-fixed type having a cover member also serving as a knob for use in electrical and electronic equipment.
As a result of recent advances in the electrical and electronic industries, there has been an increasing demand for variable resistors of the above described type, especially those of compact size which may be incorporated into various kinds of electrical and electronic equipment.
As shown by way of example in FIGS. 1 and 2, the known semi-fixed variable resistor R of the above described type generally comprises a base portion B including a substrate or base plate 1 having, for example, a rectangular configuration and made of insulating material such as bakelite and the like, a semi-circular or arcuate resistor member 2 formed on the substrate 1, a slider member 7 of similar semi-circular shape rotatably mounted on the substrate 1 to slide over the resistor member 2, and a cover member 8 mounted on the substrate 1 and also serving as a knob for rotating the slider member 7.
In the conventional variable resistor of FIGS. 1 and 2, opposite ends of the arcuate resistor member 2 are connected through electrode conductors 3a and 3b to terminal electrodes 4a and 4b, respectively. Terminal electrodes 4a and 4b are one edge of the substrate 1 and extend downwardly from said substrate 1. An opening 5 is formed in the substrate 1 at the center of an arc defined by said arcuate resistor member 2. A cylindrical sleeve 6b is formed on an electrically conductive plate 6 and is fitted into the opening 5. Conducible plate 6 has a downwardly extending common terminal 6a at one end and an upwardly extending stop member 6d at the other. The distal end of the sleeve 6b is further inserted into a fixing opening 7f provided at the central portion of the slider member 7, with subsequent outward bending or staking at the distal end 6b.sub.1 of the sleeve 6b for rotatably mounting the slider member 7 on the substrate 1. Cover member 8 includes a shaft 8c which is made, for example, of thermo-plastic resin such as nylon and extends through the hollow interior of the sleeve 6b. A projection 7a is provided on slider member 7 and is engaged by a groove 8b formed in the bottom of a concentric recess 8c formed in the cover member 8. Such that a sliding portion 7b provided on the slider member 7 may slide over the resistor member 2 responsive to rotation of the cover member 8.
In the conventional arrangement described above, the cover member 8 has been rotatably mounted on the sleeve 6b, by extending the shaft 8a of the cover member 8 through the hollow interior of the sleeve 6b and fitting a retaining ring 10 around the projecting end or tip of the shaft 8a via a washer 9 which returns the shaft 8a in place. Alternatively, or to the diameter of the tip portion of the shaft 8a is made larger than that of the internal diameter of said sleeve 6b. This may be accomplished, for example, by providing a slot (not shown) at the projecting end of the shaft 8a or by deforming the tip portion through application of heat.
The cost of manufacturing semi-fixed variable resistors of the above described type must normally be held at a minimum. However, the known practice of retaining the cover member 8 by the use of the retaining piece 10 or by the deformation of the tip portion of the shaft 8a through application of heat as described with reference to FIG. 2 requires separate parts such as the retaining piece 10, washer 9, etc. and necessitates extra processes for attaching these parts or for heat processing of the tip portion of the shaft 8a, thus resulting in high cost of the semi-fixed variable resistor. Although the disadvantage as described above may be eliminated by providing the slot at the tip portion of the shaft 8a, there arises such a problem in this case that the slot formation in a resin molding die for the cover member 8 is very difficult, if the cover member 8 is of a small size.